This invention relates to hydraulic power systems, and, more particularly, to a hydraulic power system suitable for driving a load having high inertia such as a swivelling member of a hydraulic excavator, wherein motor stopping means for preventing a hydraulic motor from rotating can be automatically actuated.
Generally, in a hydraulic power system suitable for driving, by a hydraulic motor, a load having high inertia, such as a swivelling member of a hydraulic excavator, it is necessary to detect whether the hydraulic motor is inoperative or operating at a velocity near a point at which it is inoperative, in order to automatically actuate motor stopping means for preventing the hydraulic motor from rotating without causing wear or damage thereto.
A control device provided with a detecting means is disclosed, for example, in Japanese Patent Application Laid-Open No. 98470/79 and Japanese Utility Model Application Laid-Open No. 13222/80, wherein the output shaft of a hydraulic motor for actuating a load has connected thereto an ancillary hydraulic pump, separate from a hydraulic pump, for actuating a hydraulic motor, so that the condition of rotation of the hydraulic motor can be detected by the pressure fluid delivered thereby.
In, for example, Japanese Patent Application Laid-Open No. 98471/79, another type of control device is disclosed wherein a throttle valve is mounted anteriorly or posteriorly of a hydraulic motor in a main line in which the hydraulic motor is mounted, so that the flow of hydraulic fluid in the main line can be detected by the differential pressure in the throttle valve and the condition of rotation of the hydraulic motor can thereby be detected.
In still another control device, cross-over relief valves are mounted anteriorly and posteriorly of a hydraulic motor in a main line in which the hydraulic motor is mounted and the relief condition of such valves is detected. On this device, the condition of rotation of the hydraulic motor is detected because the condition of a pumping action of the hydraulic motor is detected.
Some disadvantages are associated with all the control devices of the aforesaid type. More particularly, with a control device employing an ancillary hydraulic pump, the addition of the ancillary hydraulic pump results in a complex construction having a relatively high cost and also a loss of power. In the control device with a throttle valve in the main line, a loss of power occurs due to a pressure loss, and with the control device having cross-over relief valves, it is difficult to obtain accurate results of detection because its performance is generally governed by the characteristics of the cross-over relief valves in which the operation of the relief valves becomes unstable in a low flow rate situation.
In a hydraulic closed circuit, proposals have been made to use a control device which is operative to actuate a motor stopping means for preventing a hydraulic motor from rotating only when a control lever for controlling the displacement volume of a variable displacement hydraulic pump is in a neutral position. When a load of high inertia is driven, the hydraulic motor may continue to rotate while a relieving of the hydraulic fluid is carried out through cross-over relief valves even if the control valve is restored to the neutral position. In order to avoid operation of the motor stopping means in this condition, in, for example, Japanese Patent Application Laid-Open No. 108170/79, it is proposed to provide a predetermined time lag after the control lever is restored to the neutral position before the motor stopping means begins to operate. This time lag is obtained, for example, by providing a throttle valve in a line in which the motor stopping means is mounted. However, setting of a time lag by means of a throttle valve is not desirable because the time lag might be caused to vary by wide changes in the viscosity of the working fluid that might occur when the working fluid undergoes a temperature change. Moreover, as in the case of a swivelling member of a hydraulic excavator, where the inertia of a load applied to the hydraulic motor greatly varies depending on the condition of the load of the earth-moving machine and the position of such load, the period of time in which the relieving condition of the cross-over relief valves continues to exist would greatly vary. Thus, difficulties would be encountered in setting the time lag for allowing the motor stopping means to be actuated immediately after the hydraulic motor is essentially rendered inoperative in such a manner that the time lag suits all the conditions.